China shares land borders with many countries. Among the following, which country shares the longest land border with China?

Introduction / Context:
China is one of the world's largest countries and shares land borders with a large number of neighbours. Questions about which country shares the longest border with China test your understanding of Asian political geography and relative border lengths. This is especially important for grasping regional dynamics, security issues, and trade routes. The question asks you to identify, among four important neighbours, the one that has the longest land boundary with China.

Given Data / Assumptions:

• We are focusing on land borders only, not maritime boundaries.
• The countries listed are India, Russia, Mongolia, and Kazakhstan.
• China shares land borders with all of these countries.
• We assume approximate border lengths as reported in standard geographic references.

Concept / Approach:
China's longest land border is with Mongolia, which lies directly to its north. The China Mongolia border extends for thousands of kilometres across deserts and steppe regions. China's borders with Russia are also long but split into separate segments in the northeast and northwest and are shorter in combined length than the China Mongolia border. The China India border, though politically sensitive, is shorter than both of these. The border with Kazakhstan is significant but also shorter than that with Mongolia. Thus, among the options provided, Mongolia is the country sharing the longest land border with China.

Step-by-Step Solution:
Step 1: Identify the four neighbouring countries listed: India to the southwest, Russia to the north and northeast, Mongolia directly to the north, and Kazakhstan to the northwest of China. Step 2: Recall from maps that Mongolia stretches for a long distance along China's northern frontier. Step 3: Compare this with the China Russia border, which, while long, is divided into segments and has a smaller total length than the China Mongolia border. Step 4: Remember that the China India border is comparatively shorter, even though it is often in the news due to disputes. Step 5: Recognise that Kazakhstan's border with China is also shorter than Mongolia's, leading to the conclusion that Mongolia shares the longest land border with China.

Verification / Alternative check:
Standard tables of international boundary lengths list China's border with Mongolia as extending for roughly 4600 kilometres, making it the longest among China's land borders. The China Russia border is somewhat shorter when both eastern and western segments are combined, and the China India border is shorter still. Kazakhstan's border with China is also notably shorter than Mongolia's. This numerical comparison confirms that Mongolia, not Russia, India, or Kazakhstan, is the correct answer.

Why Other Options Are Wrong:
India has a significant and strategically important border with China, but its length is less than that of the China Mongolia border, so option A is incorrect. Russia has a long border with China, but when measured precisely, it still does not exceed the total length of the China Mongolia boundary, making option B wrong. Kazakhstan shares a border with China in Central Asia, yet its length is clearly shorter than Mongolia's extensive frontier, so option D is also incorrect. Only Mongolia has the longest continuous land border with China among the countries listed.

Common Pitfalls:
Many students assume Russia has the longest border with China because both countries are huge and share boundaries in multiple regions. Others may guess India due to frequent news reports about border disputes, without considering actual length. To avoid such mistakes, study a political map and note that Mongolia runs along a large portion of China's northern border, giving it the longest shared boundary with China.

Final Answer:
Among the given options, the country that shares the longest land border with China is Mongolia.